Abstract

Waikorohihi and Mahanga, two geysers located on Geyser Flat at Te Whakarewarewatangaoteopetauaawahiao, New Zealand, discharge chloride waters with a pH of 8.5-9.25 at 100°C. Many large (up to 10 × 8 × 5 cm) coated grains composed of amorphous silica (opal-A), that are informally termed "geyser eggs", surround the vent of Waikorohihi, whereas smaller (< 3 cm) geyser eggs are present in the proximal outflow channel of Mahanga. The geyser eggs have nuclei of detrital sinter or rhyolitic tuff. Their cortices are formed of thin concentric laminae and (or) spicular geyserite. Most cortical laminae contain a low-diversity biota that includes filamentous microbes (three types) and scattered spherical microbes. Silicified grains of Pinus radiata pollen are common in some laminae. Multiple phases of silica encrustation around the filamentous microbes disguised most features that are needed to confidently identify them. Most filaments are probably cyanobacteria. Variations in the microporosity and (or) microbe concentrations define individual cortical laminae.

Growth of the geyser eggs in the pool around Waikorohihi vent was controlled by the eruptive behavior of the geyser. The eggs are submerged by boiling water during eruptions. During dormant phases, the geyser eggs are subaerially exposed. While exposed, their surfaces may remain moist from steam or rainwater, or they may desiccate. The low microbial diversity in the geyser eggs is probably related to their periodic inundation by boiling water and the highly variable conditions. The abundant microbial laminae in the geyser eggs confirm that microbial templates are found in siliceous coated grains from almost all environmental settings in siliceous thermal-spring systems.

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